Process control monitors for interferometric modulators

ABSTRACT

Process control monitors are disclosed that are produced using at least some of the same process steps used to manufacture a MEMS device. Analysis of the process control monitors can provide information regarding properties of the MEMS device and components or sub-components in the device. This information can be used to identify errors in processing or to optimize the MEMS device. In some embodiments, analysis of the process control monitors may utilize optical measurements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/198,888, filed Aug. 5, 2005, which claims priority to U.S.Provisional Application No. 60/613,537, filed on Sep. 27, 2004, both ofwhich are incorporated herein by reference in their entirety. Thisapplication is also related to co-pending application Ser. No. ______,entitled “Process Control Monitors for Interferometric Modulators,”filed Nov. 17, 2005, attorney docket no. IRDM.152DV2; application Ser.No. ______, entitled “Process Control Monitors for InterferometricModulators,” filed Nov. 17, 2005, attorney docket no. IRDM.152DV3; andapplication Ser. No. ______, entitled “Process Control Monitors forInterferometric Modulators,” filed Nov. 17, 2005, attorney docket no.IRDM.152DV4; all of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention relates to microelectromechanical systems(MEMS).

2. Description of the Related Art

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

Errors can occur during the manufacturing of MEMS devices. Detecting theerrors and the source of the errors can present a problem in the qualitycontrol and optimization of MEMS devices. Accordingly, there is a needfor structures and methods for monitoring manufacturing processes andtheir results.

SUMMARY OF THE INVENTION

One embodiment disclosed herein includes a method of obtaininginformation regarding manufacturing processes used to manufacture amicro-electro-mechanical system (MEMS), the method including forming atleast one MEMS structure on a first side of a substrate through a seriesof deposition and patterning steps, simultaneously forming at least oneprocess control monitor on the first side of the substrate utilizing theseries of deposition and patterning steps, wherein the process controlmonitor has at least one structural difference from the MEMS structure,and detecting light reflected from the process control monitor from asecond side of the substrate opposite the first side, whereby thedetected light provides a characteristic of at least one materialdeposited or removed during the deposition and patterning steps.

Another embodiment disclosed herein includes a method of monitoringinterferometric modulator manufacturing processes, wherein themanufacturing process comprises a series of deposition and patterningsteps, the method including forming a process control monitor using theseries of deposition and patterning steps, wherein the process controlmonitor has at least one structural difference from interferometricmodulators formed by the manufacturing process and detecting opticalreflectance from the process control monitor.

Another embodiment disclosed herein includes a process control monitorfor use in monitoring interferometric modulator manufacturing processes,wherein the interferometric modulators are adapted for use in a display,the process control monitor manufactured by a process comprising atleast one step in common with steps used to manufacture theinterferometric modulators adapted for use in the display.

Another embodiment disclosed herein includes a wafer, comprising one ormore interferometric modulators adapted for use in a display and one ormore process control monitors adapted to reflect incident light andthereby provide information regarding processes used to manufacture theone or more interferometric modulators.

Another embodiment disclosed herein includes a wafer, comprising aplurality of first means for reflecting light for use in a display andsecond means for monitoring processes used to manufacture the firstmeans.

Another embodiment disclosed herein includes a display, comprising afirst wafer that comprises a plurality of interferometric modulators,wherein the first wafer is produced by a process comprising forming theplurality of interferometric modulators and at least one process controlmonitor on a second wafer and cutting the second wafer to remove theprocess control monitor and thereby produce the first wafer.

Another embodiment disclosed herein includes a method of identifying anarray of interferometric modulators as suitable for use in a display,wherein the interferometric modulators are manufactured by a processcomprising a series of deposition and patterning steps, the methodincluding forming at least one process control monitor using at leastsome of the series of deposition and patterning steps and detecting atleast one characteristic of the process control monitor.

Another embodiment disclosed herein includes a method of monitoring theextent of etching of a first material positioned between and adjacent totwo layers of other material during manufacturing of amicro-electro-mechanical system (MEMS), the method includingmanufacturing a process control monitor that comprises the two layers ofother material and the first material disposed between and adjacent tothe two layers, wherein one of the two layers comprises a hole, exposingthe hole to an etchant, and optically detecting a distance from thecenter of the hole to where the etchant has etched away the firstmaterial, whereby the distance is indicative of the extent of etching ofthe first material.

Another embodiment disclosed herein includes a wafer, comprising aplurality of structures comprising a sacrificial layer and at least onelayer above and adjacent to the sacrificial layer, wherein thestructures become interferometric modulators upon removal of thesacrificial layer, wherein the at least one layer above and adjacent tothe sacrificial layer comprises a plurality of holes through which anetchant can reach the sacrificial layer and a process control monitoralso comprising the sacrificial layer and the at least one layer aboveand adjacent to the sacrificial layer, wherein the at least one layerabove and adjacent to the sacrificial layer in the process controlmonitor comprises multiple holes, wherein the distance between the holesin the process control monitor is greater than the distance between theplurality of holes in the plurality of structures.

Another embodiment disclosed herein includes a wafer, comprising aplurality of structures comprising a sacrificial layer and at least onelayer above and adjacent to the sacrificial layer, wherein thestructures become interferometric modulators upon removal of thesacrificial layer, wherein the at least one layer above and adjacent tothe sacrificial layer comprises a plurality of holes through which anetchant can reach the sacrificial layer and a process control monitoralso comprising the sacrificial layer and the at least one layer aboveand adjacent to the sacrificial layer, wherein the at least one layerabove and adjacent to the sacrificial layer in the process controlmonitor comprises a single hole.

Another embodiment disclosed herein includes a method of manufacturing awafer having a micro-electro-mechanical system (MEMS) and a processcontrol monitor structure, the method including forming a plurality ofstructures, wherein forming the plurality of structures includes one ormore material deposition and removal steps, wherein the structurescomprise a sacrificial layer and at least one layer above and adjacentto the sacrificial layer, wherein the at least one layer above andadjacent to the sacrificial layer comprises a plurality of holes throughwhich an etchant can reach the sacrificial layer, simultaneously forminga process control monitor, wherein forming the process control monitorincludes the one or more material deposition and removal steps, whereinthe process control monitor also comprises the sacrificial layer and theat least one layer above and adjacent to the sacrificial layer, whereinthe at least one layer above and adjacent to the sacrificial layer inthe process control monitor comprises multiple holes, wherein thedistance between the holes in the process control monitor is greaterthan the distance between the plurality of holes in the plurality ofstructures, and exposing the plurality of structures and the processcontrol monitor to an etchant.

Another embodiment disclosed herein includes a wafer, comprising: amicro-electro-mechanical structure (MEMS) and means for measuring extentof etching of a material removed during manufacturing of the MEMS.

Another embodiment disclosed herein includes a process control monitorproduced by process including depositing at least three layers ofmaterial on top of each other and forming a hole in the top layer ofmaterial.

Another embodiment disclosed herein includes a method for determiningthe effect of an interferometric modulator manufacturing process oncolor reflected from interferometric modulators manufactured by theprocess, the method including manufacturing a plurality ofinterferometric modulators comprising posts that support a firstmechanical membrane, manufacturing a process control monitor etaloncomprising posts that support a second mechanical membrane, wherein theposts in the process control monitor are present in higher density thanthe posts in the plurality of interferometric modulators, and detectinglight reflected from the process control monitor etalon, whereby thedetected light provides an indication of the depth of an interferometriccavity in the plurality of interferometric modulators.

Another embodiment disclosed herein includes a process control monitorfor monitoring the effect of a process for manufacturing interferometricmodulators on color reflected by those interferometric modulators,comprising a test etalon that comprises a higher density of postssupporting a mechanical membrane in the test etalon than ininterferometric modulators produced by the process.

Another embodiment disclosed herein includes a wafer, comprising aplurality of interferometric modulators adapted for use in a display anda process control monitor that comprises an etalon having a higherdensity of posts supporting a mechanical membrane than in the pluralityof interferometric modulators.

Another embodiment disclosed herein includes a process control monitor,omc;idomg an etalon having a conductive partial mirror and a conductivemechanical membrane comprising a mirror, wherein the mechanical membraneis separated from the partial mirror by a plurality of posts, whereinthe density of posts is high enough such that the mechanical membranecannot collapse toward the partial mirror when a voltage is appliedbetween the partial mirror and the mechanical membrane.

Another embodiment disclosed herein includes a method of manufacturing acombined micro-electro-mechanical system (MEMS) and process controlmonitor structure, the method including forming a MEMS structure,wherein forming the MEMS structure includes one or more materialdeposition and patterning steps, wherein the MEMS structure comprises afirst mechanical membrane supported by a first plurality of posts andsimultaneously forming a process control monitor, wherein forming theprocess control monitor includes the one or more material deposition andpatterning steps, the process control monitor comprising a secondmechanical membrane supported by a second plurality of posts, whereinthe second plurality of posts are present in a higher density than thefirst plurality of posts.

Another embodiment disclosed herein includes a wafer, comprising aplurality of first means for reflecting light for use in a display andsecond means for stably reflecting light having substantially the samecolor as reflected from at least one of the second means.

Another embodiment disclosed herein includes a process control monitorproduced by a process that includes forming a partial mirror, forming amechanical membrane, and forming a plurality of posts supporting themechanical membrane and separating the mechanical membrane from thepartial mirror, wherein the density of posts is high enough such thatthe mechanical membrane cannot collapse toward the partial mirror when avoltage is applied between the partial mirror and the mechanicalmembrane.

Another embodiment disclosed herein includes a method of monitoringdeposition of material deposited during manufacturing of amicro-electro-mechanical system (MEMS), the method including forming aprocess control monitor that consists of at least three layers ofmaterial deposited during the manufacturing, wherein the at least threelayers of material is less than the number of layers deposited duringmanufacturing of the MEMS, wherein the at least three layers of materialform an etalon and detecting light reflected from the etalon, wherebyinformation regarding properties of the at least three layers isobtained.

Another embodiment disclosed herein includes a wafer, comprising aplurality of interferometric modulators adapted for use in a display anda non-modulating interferometer.

Another embodiment disclosed herein includes a method of monitoringdeposition of material deposited during manufacturing of amicro-electro-mechanical system (MEMS), the method including forming aprocess control monitor comprising one or more layers of materialdeposited during the manufacturing, wherein the number of layers ofmaterial in the process control monitor is less than the number oflayers deposited during manufacturing of the MEMS and detecting thereflectance of the process control monitor, whereby the reflectanceprovides information regarding properties of the layers in the processcontrol monitor.

Another embodiment disclosed herein includes a wafer, comprising aplurality of interferometric modulators adapted for use in a display,the interferometric modulators comprising a plurality of material layersand a process control monitor comprising one or more of the materiallayers, wherein the process control monitor comprises less than all ofthe plurality of material layers.

Another embodiment disclosed herein includes a method of manufacturing acombined micro-electro-mechanical system (MEMS) and process controlmonitor structure, the method including forming a MEMS structure,wherein forming the MEMS structure includes one or more materialdeposition and patterning steps and simultaneously forming a processcontrol monitor, wherein forming the process control monitor includesthe one or more material deposition and patterning steps, wherein theprocess control monitor comprises less than all components present inthe MEMS structure.

Another embodiment disclosed herein includes a wafer produced by aprocess that includes depositing and patterning a series of materiallayers on a substrate to form a MEMS structure and simultaneouslydepositing and patterning a series of material layers on the substrateto form a process control monitor, wherein the process control monitorcomprises less than all components present in the MEMS structure.

Another embodiment disclosed herein includes a method of measuringthicknesses of layers deposited during manufacture of amicro-electro-mechanical system (MEMS), the method including forming astructure that comprises two or more layers successively deposited ontop of each other, wherein the layers are formed using a process that isused for forming those layers during manufacture of the MEMS, whereinthe layers are patterned such that at least two steps are formed in aprofile of the structure and measuring the height of the steps bysweeping a profilometer across the structure.

Another embodiment disclosed herein includes a process control monitorfor measuring the thicknesses of a plurality of layers deposited duringmanufacturing of an interferometric modulator, comprising the layersstacked on top of each other so as to form at least two steps in aprofile of the process control monitor.

Another embodiment disclosed herein includes a wafer, comprising aplurality of interferometric modulators adapted for use in a display,the interferometric modulators comprising a plurality of material layersand a process control monitor comprising the plurality of materiallayers stacked on top of each other so as to form at least two steps ina profile of the process control monitor.

Another embodiment disclosed herein includes a method of manufacturing acombined micro-electro-mechanical system (MEMS) and process controlmonitor structure, the method including forming a MEMS structure,wherein forming the MEMS structure includes one or more materialdeposition and patterning steps, wherein the MEMS structure comprises aplurality of layers and simultaneously forming a process controlmonitor, wherein forming the process control monitor includes the one ormore material deposition and patterning steps, wherein the processcontrol monitor comprises the plurality of layers so as to form at leasttwo steps in a profile of the process control monitor.

Another embodiment disclosed herein includes a wafer, comprising aplurality of first means for reflecting light for use in a display andsecond means for measuring thickness of at least one material depositedduring manufacture of the first means.

Another embodiment disclosed herein includes a wafer produced by aprocess comprising depositing and patterning a series of material layerson a substrate to form a MEMS structure and simultaneously depositingand patterning the series of material layers on the substrate to form aprocess control monitor, wherein layers of material remaining in theprocess control monitor after the patterning form at least two steps ina profile of the process control monitor.

Another embodiment disclosed herein includes a method of testing aprocess used to manufacture a polychromatic interferometric modulatordisplay, wherein different color interferometric modulators in thedisplay are manufactured by forming different depth gaps between apartial reflector and a reflective mechanical membrane, wherein thedepths of the gaps are determined by deposition of one or moresacrificial layers, wherein the depth of at least one gap is determinedby deposition of a plurality of sacrificial layers, the method includingforming a process control monitor that comprises the one or moresacrificial layers, wherein at least one region of the process controlmonitor comprises the plurality of sacrificial layers deposited on topof each other, measuring a profile of the process control monitor, anddetermining a cumulative thickness of the plurality of sacrificiallayers from the profile.

Another embodiment disclosed herein includes a process control monitorfor use in testing a process used to manufacture a polychromaticinterferometric modulator display, wherein different colorinterferometric modulators in the display are manufactured by formingdifferent depth gaps between a partial reflector and a reflectivemechanical membrane, wherein the depths of the gaps are determined bydeposition of one or more sacrificial layers, wherein the depth of atleast one gap is determined by deposition of a plurality of sacrificiallayers, the process control monitor comprising a plurality of materiallayers on top of each other, wherein one region of the process controlmonitor includes a single sacrificial layer, a second region of theprocess control monitor includes two sacrificial layers on top of eachother, and a third region of the process control monitor includes threesacrificial layers on top of each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIG. 5A illustrates one exemplary frame of display data in the 3×3interferometric modulator display of FIG. 2.

FIG. 5B illustrates one exemplary timing diagram for row and columnsignals that may be used to write the frame of FIG. 5A.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a top view of a wafer comprising a MEMS structure and multipleprocess control monitors.

FIG. 9 is a cross section of layers deposited during manufacture of aninterferometric modulator.

FIG. 10A is a cross section of layers in an etalon-based process controlmonitor for use in monitoring a process used to manufacture theinterferometric modulator of FIG. 9.

FIG. 10B is a cross section of layers in another etalon-based processcontrol monitor for use in monitoring a process used to manufacture theinterferometric modulator of FIG. 9.

FIG. 10C is a cross section of layers in another etalon-based processcontrol monitor for use in monitoring a process used to manufacture theinterferometric modulator of FIG. 9.

FIG. 10D is a cross section of layers in another etalon-based processcontrol monitor for use in monitoring a process used to manufacture theinterferometric modulator of FIG. 9.

FIG. 11A is a cross section of layers in a non-etalon-based processcontrol monitor for use in monitoring a process used to manufacture theinterferometric modulator of FIG. 9.

FIG. 11B is a cross section of layers in another non-etalon-basedprocess control monitor for use in monitoring a process used tomanufacture the interferometric modulator of FIG. 9.

FIG. 11C is a cross section of layers in another non-etalon-basedprocess control monitor for use in monitoring a process used tomanufacture the interferometric modulator of FIG. 9.

FIG. 11D is a cross section of layers in another non-etalon-basedprocess control monitor for use in monitoring a process used tomanufacture the interferometric modulator of FIG. 9.

FIG. 11E is a cross section of layers in another non-etalon-basedprocess control monitor for use in monitoring a process used tomanufacture the interferometric modulator of FIG. 9.

FIG. 11F is a cross section of layers in another non-etalon-basedprocess control monitor for use in monitoring a process used tomanufacture the interferometric modulator of FIG. 9.

FIG. 11G is a cross section of layers in another non-etalon-basedprocess control monitor for use in monitoring a process used tomanufacture the interferometric modulator of FIG. 9.

FIG. 12 is a top view of a wafer comprising an interferometric modulatorarray and process control monitors used to monitor release etching andcolor reflected from the interferometric modulators.

FIG. 13A is a top view of a process control monitor that can be used tomonitor release etching.

FIG. 13B is a top view of another process control monitor that can beused to monitor release etching.

FIG. 14 is a cross section of a process control monitor that can be usedto measure the thickness of layers in interferometric modulators.

FIG. 15 is a cross section of another embodiment of a process controlmonitor that can be used to measure the thickness of layers in theprocess control monitor.

FIG. 16 is a cross section of yet another embodiment of a processcontrol monitor that can be used to measure the thickness of layers inthe process control monitor.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

Manufacturing of MEMS devices typically involve the formation of severallayers of material having structures and thicknesses formed by using aseries of material deposition, patterning, and etching steps. It can bedifficult to diagnose from the final MEMS device any errors thatoccurred during the processing of given layers in the device.Furthermore, it can be difficult to determine from the final devicewhich specific parameters, such as film thicknesses, should be adjustedin order to optimize the device for its intended use. Accordingly, thereis a need for structures and methods that can be used to monitor theresult of specific processing steps. Therefore, in various embodiments,process control monitors are provided that are constructed using atleast some of the same processes used to manufacture MEMS devices.Analysis of the process control monitors provide information regardingindividual components or sub-sets of components that make up the MEMSdevice.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise of several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. In some embodiments, the layers are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the cavity 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5B illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a panel or display array (display) 30. The cross section ofthe array illustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. ForMEMS interferometric modulators, the row/column actuation protocol maytake advantage of a hysteresis property of these devices illustrated inFIG. 3. It may require, for example, a 10 volt potential difference tocause a movable layer to deform from the relaxed state to the actuatedstate. However, when the voltage is reduced from that value, the movablelayer maintains its state as the voltage drops back below 10 volts. Inthe exemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the relaxed or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be relaxed areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putthem in. After being written, each pixel sees a potential differencewithin the “stability window” of 3-7 volts in this example. This featuremakes the pixel design illustrated in FIG. 1 stable under the sameapplied voltage conditions in either an actuated or relaxed pre-existingstate. Since each pixel of the interferometric modulator, whether in theactuated or relaxed state, is essentially a capacitor formed by thefixed and moving reflective layers, this stable state can be held at avoltage within the hysteresis window with almost no power dissipation.Essentially no current flows into the pixel if the applied potential isfixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4, 5A, and 5B illustrate one possible actuation protocol forcreating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustratesa possible set of column and row voltage levels that may be used forpixels exhibiting the hysteresis curves of FIG. 3. In the FIG. 4embodiment, actuating a pixel involves setting the appropriate column to−V_(bias), and the appropriate row to +ΔV, which may correspond to −5volts and +5 volts respectively Relaxing the pixel is accomplished bysetting the appropriate column to +V_(bias), and the appropriate row tothe same +×V, producing a zero volt potential difference across thepixel. In those rows where the row voltage is held at zero volts, thepixels are stable in whatever state they were originally in, regardlessof whether the column is at +V_(bias), or −V_(bias). As is alsoillustrated in FIG. 4, it will be appreciated that voltages of oppositepolarity than those described above can be used, e.g., actuating a pixelcan involve setting the appropriate column to +V_(bias), and theappropriate row to −ΔV. In this embodiment, releasing the pixel isaccomplished by setting the appropriate column to −V_(bias), and theappropriate row to the same −ΔV, producing a zero volt potentialdifference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding, and vacuum forming. In addition, the housing 41 may be madefrom any of a variety of materials, including but not limited toplastic, metal, glass, rubber, and ceramic, or a combination thereof. Inone embodiment the housing 41 includes removable portions (not shown)that may be interchanged with other removable portions of differentcolor, or containing different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43 which is coupled to a transceiver 47. Thetransceiver 47 is connected to the processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g. filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to the arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

Processor 21 generally controls the overall operation of the exemplarydisplay device 40. The processor 21 receives data, such as compressedimage data from the network interface 27 or an image source, andprocesses the data into raw image data or into a format that is readilyprocessed into raw image data. The processor 21 then sends the processeddata to the driver controller 29 or to frame buffer 28 for storage. Rawdata typically refers to the information that identifies the imagecharacteristics at each location within an image. For example, suchimage characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, driver controller 29is a conventional display controller or a bi-stable display controller(e.g.; an interferometric modulator controller). In another embodiment,array driver 22 is a conventional driver or a bi-stable display driver(e.g., an interferometric modulator display). In one embodiment, adriver controller 29 is integrated with the array driver 22. Such anembodiment is common in highly integrated systems such as cellularphones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

Power supply 50 can include a variety of energy storage devices as arewell known in the art. For example, in one embodiment, power supply 50is a rechargeable battery, such as a nickel-cadmium battery or a lithiumion battery. In another embodiment, power supply 50 is a renewableenergy source, a capacitor, or a solar cell, including a plastic solarcell, and solar-cell paint. In another embodiment, power supply 50 isconfigured to receive power from a wall outlet.

In some implementations control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some cases control programmabilityresides in the array driver 22. Those of skill in the art will recognizethat the above-described optimization may be implemented in any numberof hardware and/or software components and in various configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections are herein referred to as support posts. Theembodiment illustrated in FIG. 7D has support post plugs 42 upon whichthe deformable layer 34 rests. The movable reflective layer 14 remainssuspended over the cavity, as in FIGS. 7A-7C, but the deformable layer34 does not form the support posts by filling holes between thedeformable layer 34 and the optical stack 16. Rather, the support postsare formed of a planarization material, which is used to form supportpost plugs 42. The embodiment illustrated in FIG. 7E is based on theembodiment shown in FIG. 7D, but may also be adapted to work with any ofthe embodiments illustrated in FIGS. 7A-7C as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields some portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34 and the busstructure 44. This allows the shielded areas to be configured andoperated upon without negatively affecting the image quality. Thisseparable modulator architecture allows the structural design andmaterials used for the electromechanical aspects and the optical aspectsof the modulator to be selected and to function independently of eachother. Moreover, the embodiments shown in FIGS. 7C-7E have additionalbenefits deriving from the decoupling of the optical properties of thereflective layer 14 from its mechanical properties, which are carriedout by the deformable layer 34. This allows the structural design andmaterials used for the reflective layer 14 to be optimized with respectto the optical properties, and the structural design and materials usedfor the deformable layer 34 to be optimized with respect to desiredmechanical properties.

Process Control Monitors

Many MEMS manufacturing processes consist of a series of materialdeposition and patterning steps. Various materials may be seriallydeposited on a substrate to form layers. Patterning with materialetching between deposition steps may be used to structurally tailor thedeposited material to achieve the desired MEMS structure. The multiplelayer approach to MEMS manufacturing and the small scale of thestructures created presents problems in trying evaluate whether themanufacturing process has produced structures and layers of materialshaving the desired properties. Accordingly, in one embodiment, processcontrol monitors are provided that may be used to evaluate the resultsof various manufacturing processes. In some embodiments, the processcontrol monitors are produced using at least some of the samemanufacturing steps used to manufacture a MEMS device. Evaluation ofthese process control monitors may then be used to determine propertiesof various materials and structures formed during those manufacturingsteps. In some embodiments, the process control monitors are producedusing the same set of material deposition and patterning steps usedduring the manufacturing. The process control monitors may bestructurally tailored by applying different patterns to the processcontrol monitor than is applied to the MEMS structure. For example, onelayer of material present in the MEMS structure may be completely absentin the process control monitor by patterning the process control monitorin such a way that the entire layer deposited is etched away during anetching step. Similarly, in other embodiments, a layer of materialnormally etched away during manufacture of the MEMS structure may remainin the process control monitor.

In some embodiments, information from the process control monitor may beobtained through optical means. For example, light reflected from theprocess control monitor may contain information regarding the materialspresent in the process control monitor. Those of skill in the art willappreciate other methods of evaluating process control monitors such aslaser scanning, microscopy including optical, electron, and x-raymicroscopy, and spectroscopy. In one embodiment, the light reflected isdetected with a photodetector to obtain the intensity of reflectedlight. This information may be used to determine the reflectance andtransmittance of materials in the process control monitor. Theseproperties may in turn provide information regarding the thicknesses ofmaterials in the process control monitor. For example, the amount ofreflectance from an inherently reflective material will provide ameasure of its thickness. In one embodiment, a Minolta® reflectometer isused. In another embodiment, the light reflected from the processcontrol monitor is measured with a spectrometer to obtain the wavelengthdependence of the reflected light. This wavelength dependence canprovide information regarding the absorptive properties of the materialsin the process control monitor and the index of refraction of thematerials. Furthermore, because MEMS devices often contain reflectivesurfaces in close proximity, reflected light may undergo constructiveand destructive interference (e.g., the MEMS device may contain one ormore etalon). Therefore, the wavelength dependence of the reflectedlight can provide information regarding the relative positioning ofreflective surfaces in the MEMS. In one embodiment, a measured spectrumis fit to a model spectrum predicted to be reflected from an etalon inorder to determine properties such as the depth of the etalon. In someembodiments, a colorimeter is used to measure the color of lightreflected from the process control monitor. As used herein, an “etalon”refers to two surfaces that are at least partially reflective positionedsuch that light may enter through one surface and be reflected betweenthe two surfaces multiple times before reflecting back through the samesurface. The multiple reflectances can lead to destructive andconstructive interference at various wavelengths, allowing for thefiltering of optical wavelengths.

In one embodiment, a transparent substrate may be used to support theprocess control monitor. Such a substrate enables optical detection fromthe side opposite the deposition side. Thus, in some cases, lowerdeposited materials may be probed where they could not otherwise be(e.g., where the upper layers include a highly reflective layer). Inother embodiments, a process control monitor is optically probed fromthe side of material deposition.

In one embodiment, with reference to FIG. 8, process control monitors100, 102, and 104 may be formed on the same substrate 106 at the sametime as the MEMS device 108 is being formed. As discussed above, all ofthe substrate 106 may be exposed to the same material deposition andpatterning steps, however, different patterns may be applied to form theprocess control monitors 100, 102, and 104. For example, the patternapplied to the process control monitors 100, 102, and 104 during apatterning step may be different then the pattern applied to the MEMSdevice 108 during a corresponding patterning step. The patterning stepsmay include any suitable patterning technique in the art (e.g.photolithography). Any number of different process control monitors 100,102, and 104 may be formed on the substrate. The integrated wafer 110depicted in FIG. 8 allows the probing of the processes applied duringthe manufacturing of the specific MEMS device 108. Thus, any anomalousresults can be quickly identified before the MEMS device 108 is testedelectrically or incorporated into a packaged device, thereby avoidingadditional expense. In some embodiments, the process control monitors100, 102, and 104 may also be probed after manufacture of the MEMSdevice 108. In one embodiment, the MEMS device 108 consists of an arrayof interferometric modulators suitable for use in a display. In someembodiments, the process control monitors on the substrate 106 arelabeled during manufacturing.

Etalon Based Process Control Monitors

As noted above, in some embodiments, process control monitors areconstructed such that they contain at least one etalon. The spectrum oflight reflected from the etalon may then be detected and fit to anetalon model to determine properties of the process control monitor, andhence properties of analogous structures in the MEMS device. In someembodiments, the process control monitors are formed by the samematerial deposition steps as the MEMS device and thus contain at leastsome of the material layers found in the MEMS device. In someembodiments, the number of layers found in the process control monitorsis less than the number found in the MEMS device.

One set of examples of etalon based process control monitors arestructures that contain less than all of the layers found an ininterferometric modulator but nonetheless still contain an etalon. FIG.9 depicts an example of materials that may be deposited duringmanufacture of an interferometric modulator. First, a layer ofindium-tin-oxide (ITO) 154 is deposited onto a transparent substrate152. The ITO 154, which is a transparent conductor, provides aconductive plate so that a voltage can be applied between the movablemirror in the interferometric modulator and the plate. In oneembodiment, the ITO is about 500 Åthick. Next, a layer of chrome 150 isdeposited. In one embodiment, the chrome 150 is relatively thin (in oneembodiment, approximately 70 Å), allowing it to act as a partialreflector. Alternatively, the chrome layer 150 may be deposited onto thesubstrate 152 followed by the ITO layer 154. Next, a dielectric layer156/158 is deposited. The dielectric layer may consist of one or moreoxides. In some embodiments, the oxide layer 156/158 may be a compositelayer. For example, a relatively thick layer of SiO₂ 156 (in oneembodiment, approximately 450 Å) may be deposited followed by a thinlayer of Al₂O₃ 158 (in one embodiment, approximately 70 Å) to protectthe SiO₂ 156. In some embodiments, three or more oxide layers may beused (e.g., Al₂O₃—SiO₂—Al₂O₃). The oxide layer 156/158 provides aninsulating layer between the movable mirror and the chrome 150. Thethickness of the layer determines the interference properties of theinterferometric modulator, particularly when it is in an actuated state.In the next step, a sacrificial layer 160 is deposited (in oneembodiment, approximately 2000 Å). The sacrificial layer provides aspace filling material that can be easily etched away without affectingthe other materials. In one embodiment, the sacrificial layer 160 ismolybdenum. Other examples of suitable materials for the sacrificiallayer include polysilicon, amorphous silicon, or photoresist. In thelast step of manufacturing, the sacrificial layer 160 will be etchedaway to create an air gap between the movable mirror and the oxide layer156,158. Patterning and etching of the sacrificial layer 160 may be usedto create holes and trenches in the layer for the formation of posts andrails that will support the movable mirror. Planar material 162 may beapplied to fill the holes and form the posts. Finally, the mechanicalmembrane 164/166 containing the movable mirror is formed. In oneembodiment, the mechanical membrane 164/166 is formed by an aluminumlayer 164 (in one embodiment, approximately 500 Å) followed by a nickellayer (in one embodiment, approximately 1450 Å) 166. In someembodiments, an additional aluminum layer is added on top of the nickellayer to provide better adhesion of photoresist used during patterning.After etching away the sacrificial layer 160 in the structure depictedin FIG. 9, an interferometric modulator similar to that depicted in FIG.7A is obtained. In some embodiments, a dark mask layer may be added tothe transparent substrate 152 prior to addition of the other layers. Thedark mask layer may be patterned to reduce reflection from portions ofthe structure such as posts or rails. In some embodiments, the dark masklayer includes a MoCr layer and an oxide layer. Those of skill in theart will appreciate that patterning and etching steps in addition tothose mentioned here may be used to form an interferometric modulator.Furthermore, it will be appreciated that other structures ofinterferometric modulators are possible, as for example depicted inFIGS. 7B-7E.

Examples of etalon based process control monitors containing some of thematerial layers discussed above are depicted in FIGS. 10A-10D. Theprocess control monitor depicted in FIG. 10A contains the ITO 154,chrome 150, oxide 156/158, and mechanical membrane 164/166 layersdeposited on top of each other onto the substrate 152. The partiallyreflective chrome layer 150 and the reflective mechanical membrane164/166 form an etalon whose reflectance may be measured from the bottomside of the substrate 152. Analyzing the spectrum of light reflectedfrom this etalon or its color can provide an indication of the combinedthickness of the oxide 156/158 layers and their index of refractions andthe thickness and reflectivity of the chrome 150 layer. It will also beappreciated that this configuration approximates that obtained when aninterferometric modulator is in an actuated state (i.e., the mirror iscollapsed against the oxide layer). Therefore, evaluating these processcontrol monitors will provide an indication of whether interferometricmodulators produced by the process used will have the desired actuatedspectral characteristics.

The process control monitor depicted in FIG. 10B consists of the ITO154, chrome 150, oxide 156/158, and sacrificial layer 160. As mentionedabove, the sacrificial layer 160 may be molybdenum, which is inherentlyreflective. Accordingly, an etalon is formed by the partially reflectivechrome layer 150 and the reflective sacrificial layer 160. In additionto providing the same parameters regarding the oxide 156/158 layers andthe actuated interferometric modulator state as discussed above,reflectance from this process control monitor may provide informationregarding the sacrificial layer 160. For example, reflectance from thesacrificial layer 160 will depend on the thickness of the sacrificiallayer 160. In some embodiments, the sacrificial layer 160 is removed byetching and the remaining ITO 154, chrome 150, and oxide 156/158 layersanalyzed to determined if the sacrificial layer 160 has interacted withany of the remaining layers.

The process control monitor depicted in FIG. 10C contains the ITO 154,chrome 150, oxide 156/158, planar 162, and mechanical membrane 164/166layers. An etalon is formed by the chrome 150 and mechanical membrane164/166 layers. Analyzing the spectrum of reflected light and comparingit to the results obtained for the process control monitor in FIG. 10Acan provide the index of refraction of the planar material and itsthickness. Furthermore, the optical response from this process controlmonitor will approximate that caused by the areas of an interferometricmodulator array where there are posts or rails.

The process control monitor depicted in FIG. 10D contains the ITO 154,chrome 150, planar 162, and mechanical membrane 164/166 layers. Anetalon is formed by the chrome 150 and mechanical membrane 164/166layers. Analyzing the spectrum of reflected light can provide the indexof refraction of the planar material 162 as well as the thickness of theplanar 162 material. Comparison with the process control monitor of FIG.10D can provide information regarding the oxide layers 156/158 (e.g.,indices of refraction and thickness).

When the etalon based process control monitors described above areformed by the same deposition and patterning steps as used tomanufacture interferometric modulators, as for example when it is formedon the same substrate 106 as an interferometric modulator array 108 (seeFIG. 8), then appropriate patterning may be applied so that layers thatare not desired in the process control monitor are etched away. Forexample, in the process control monitor depicted in FIG. 10A, thesacrificial layer 160 and planar 162 material deposited duringmanufacturing may be etched away. In some embodiments, it may bedesirable to protect regions of the process control monitors to preventetching away of layers during processing. For example, deposited planarmaterial or material from the mechanical membrane 164/166 may bepatterned so that it remains on the edges of the process control monitorto protect the sacrificial layer 160 during the release etch if it isdesirable to have a process control monitor containing the sacrificiallayer 160.

Those of skill in the art will appreciate many other combinations oflayers deposited in a process control monitor whose optical properties(e.g., interference properties) may provide information about thecorresponding material formed during manufacturing of a MEMS device.

Non-Etalon Based Process Control Monitors

In some embodiments, process control monitors are constructed that donot contain two reflective surfaces forming an etalon. In these processcontrol monitors, information regarding the materials in the monitorscan be obtained through reflectance and/or transmittance measurements.These reflectance and/or transmittance values may be correlated to filmthicknesses. In some embodiments, the process control monitors areformed by the same material deposition steps as the MEMS device and thuscontain at least some of the material layers found in the MEMS device.In some embodiments, the number of layers found in the process controlmonitors is less than the number found in the MEMS device. Thereflectance and/or transmittance characteristics of these structures mayhelp identify any errors that occurred during processing of the elementsincluded in the process control monitor structures. These processcontrol monitor structures may be evaluated using any suitable detectorsuch as a reflectometer, photodetector, spectrometer, or colorimeter. Inone embodiment, the reflectance of the film is measured using aspherical integrator and reflectometer. These process control monitorstructures enable the processing of individual elements in the MEMSstructures to be monitored to determine any errors and to optimize themanufacturing process.

FIGS. 11A-11G depict one set of examples of non-etalon based processcontrol monitors that contain less than all layers of material depositedduring manufacture of interferometric modulators such as depicted inFIG. 9. The process control monitor in FIG. 11A consists of the ITOlayer 154 and the chrome layer 150 deposited onto the substrate 152. Thereflectivity of this process control monitor provides an indication ofthe thickness of the chrome layer 150 and the transparency of the ITOlayer 154. In order for the chrome layer 150 to act as a partiallyreflective mirror in an interferometric modulator, the film making upthe partial reflector may be very thin. For example, the film may have athickness of about 70 Å. The thickness of such thin films are difficultto measure and verify. Therefore, in one embodiment, the thickness ofthe chrome layer 150 is determined by measuring the reflectance of thelayer in the process control monitor of FIG. 11A. As the thickness ofthe film increases, so will the reflectance. Therefore, by calibratingfilm thickness with measured reflectance for a given material, thethickness can be easily determined from a measured reflectance. Theoptical properties of the process control monitor of FIG. 11A alsoapproximate the optical properties observed in an interferometricmodulator array between columns where the mechanical membrane and oxidelayers have been removed. Accordingly, these process control monitorscan be used to determine if the intercolumn properties are acceptablefor using the array as a display.

In another embodiment, a process control monitor containing only thechrome layer 150 on the substrate 152 may be used to determine thereflectance, and hence the thickness, of the chrome layer 150.Measurements of this process control monitor may be compared to thoseobtained for the process control monitor depicted in FIG. 11A todetermine the optical properties of the ITO layer 154. For example,reflectance from the surface of the ITO layer 154 may be proportional tothe ratio of the reflectance from the two process control monitors. Insome embodiments, a chrome-only process control monitor may manufacturedon a wafer separate from that used to manufacture the interferometricmodulators if the processing conditions for the interferometricmodulators cannot be used to create a chrome-only layer.

FIG. 11B depicts another embodiment of a non-etalon based processcontrol monitor structure that consists of the ITO layer 154, chromelayer 150, and oxide layer 156/158. This structure may be used tomeasure the optical characteristics of the ITO-chrome-oxide combination.For example, measuring the transmittance through the process controlmonitor provides an indication of the combined attenuation caused by theITO layer 154, chrome layer 150, and oxide layer 156/158. Comparison ofthe measurements of this process control monitor structure with themeasurements of the process control monitor in FIG. 11A can be used toisolate the optical properties of the oxide layer 156/158. In additionto providing information regarding the optical characteristics of theoxide layer 156/158, the comparison can also be used to determine thethickness of the oxide layer 156/158 (e.g., a lower transmittance willindicate a thicker oxide layer 156/158). The optical properties of theprocess control monitor in FIG. 11B also approximates those observed inan interferometric modulator array in the area of the etch release holesin the mechanical membrane.

FIG. 11C depicts another embodiment of a process control monitorstructure that consists of the mechanical membrane layer 164/166. Thisprocess control monitor may be used to isolate and measure thereflective properties of the mechanical membrane layer 164/166.

FIG. 11D depicts still another embodiment of a process control monitorstructure consisting only of the sacrificial layer 160 deposited ontothe substrate 152. This process control monitor may be used to measurecharacteristics of the sacrificial layer 160 alone, including itsthickness. This process control monitor may be analyzed prior to anyrelease etch. Alternatively a layer of a protective material may bedeposited over the sacrificial layer 160 to protect it during a releaseetch.

FIG. 11E depicts another embodiment of a process control monitor havingoxide layers 156/158, planar material 162, and mechanical membrane layer164/166. The reflectance from this process control monitor approximatesthat observed in an interferometric modulator array between rows wherecuts in the ITO 154 and chrome 158 layers have been made.

FIG. 11F depicts an embodiment of a process control monitor having theITO layer 154, chrome layer 150, and mechanical membrane layer 164/166.Because the chrome layer 150 and mechanical membrane layer 164/166 willtogether act is a reflector, the reflectance from this process controlmonitor can provide information regarding the transparency, thickness,and index of refraction of the ITO layer 154. Furthermore, thereflectance from this process control monitor may be compared with thatfor FIG. 11A to isolate the optical properties of the chrome layer 150.In other words, the results from testing this process monitor may beused to subtract the optical effects of the ITO layer 154 in the processcontrol monitor of FIG. 11A.

FIG. 11G depicts still another embodiment of a process control monitorthat comprises the oxide layer 156/158 and the mechanical membrane layer164/166. Because the mechanical membrane layer 164/166 acts as a strongreflector, this process control monitor may be used to determine thetransparency, thickness, and index of refraction of the oxide layer156/158.

As for the etalon based process control monitors, the non-etalon basedprocess control monitors described above may be formed by the samedeposition and patterning steps as used to manufacture theinterferometric modulators. Appropriate patterning may be applied sothat layers that are not desired in the process control monitor areetched away. In addition, appropriate protection against etching may beapplied.

Those of skill in the art will appreciate many other combinations oflayers deposited in a process control monitor whose optical properties(e.g., reflectance and/or transmittance) may provide information aboutthe corresponding material formed during manufacturing of a MEMS device.

Release Etch Process Control Monitors

The rate and extent of the release etch process during MEMS manufacturemay be monitored using a release etch or spatial process controlmonitor. FIG. 12 depicts a wafer 200 containing an interferometricmodulator array 202 and a series of process control monitors 204, 206,and 208. The interferometric modulator array 202 contains a number ofposts 210 and rails 212 to support the mechanical membrane. A series ofetch holes 214 are formed into the mechanical membrane so that etchantcan reach the sacrificial layer during the release etch. For themanufacturing to be successful, the sacrificial layer should becompletely removed from the array region. Accordingly, in oneembodiment, process control monitors are provided to monitor the rateand extent of release etching.

One such process control monitor is depicted in process control monitor206. This process control monitor 206 consists of the sameinterferometric modulator structure as present in the array 202,however, only a single hole 216 is patterned into the mechanicalmembrane. The distance between the hole 216 and the edges of the processcontrol monitor 206 is greater than the distance between the holes 214in the interferometric modulator array 202. Because the process controlmonitor 206 contains only a single hole 216 as opposed to multiple holes214, not all of the sacrificial layer can be removed from the processcontrol monitor 206 in the amount of time it takes the release etchantto remove the entire sacrificial layer in the array 202. As the etchingin the process control monitor 206 proceeds, the area of the processcontrol monitor where the sacrificial layer has been removed willcontrast in color from the areas where the etchant has not yet reachedas observed from the side of the substrate opposite the process side. Incases where a reflective sacrificial layer is used (e.g., molybdenum),this contrast is due to the different etalons formed. Where thesacrificial layer is still present, an etalon will be formed between thechrome layer and the reflective sacrificial layer. Where the sacrificiallayer has been removed, an etalon will be formed between the chromelayer and the reflective mechanical membrane. Thus, the color observedwhere the sacrificial layer has been removed will approximate the colorof an unactuated interferometric modulator (e.g., a bright state) whilethe color observed where the sacrificial layer remains will approximatethe color of an actuated interferometric modulator (e.g., a dark state).The distance from the center of the hole 216 to the boundary of colorchange (e.g., the radius) will provide a measure of the extent ofetching. This process control monitor may be used to measure the rateand extent of etching either during the process itself (i.e., in-situ)or after its completion.

A similar etch-release process control monitor is depicted by theprocess control monitor 208. In this process control monitor, multipleholes 218 are formed in the mechanical membrane, however, the distancebetween each hole 218 is greater than the distance between the holes 214in the interferometric modulator array 202. Thus, etching in the processcontrol monitor 208 will be incomplete after the entire sacrificiallayer has been removed from the interferometric modulator array 202. Adistance indicating the extent of etching may be measured from thecenter of each hole 218 in the process control monitor 208.

Etch-release process control monitors as described above may take on anysuitable shape. For example, instead of a structure similar to thatfound in the interferometric modulator array, the process controlmonitor may consist of a strip shape 250 with one or more holes 252 inthe mechanical membrane, as depicted in FIG. 13A. The extent of etchingmay then be measured by determining the linear distance from the holes252 along the strip 250 to where etching has extended. In anotherembodiment, depicted in FIG. 13B, holes having the shape of rectangularslots 254 are formed into the strip 250 instead of holes having acircular shape. In some embodiments, a plurality of slots 254 areprovided having varying widths (e.g., 3 μm, 4 μm, 5 μm).

In some embodiments, planarization or other protective material may bepatterned around the edges of the process control monitor to provide aseal to protect against the release etchant from reaching thesacrificial layer from the edges. Accordingly, the sacrificial layerwill only be removed by etchant entering the etch release holes. In someembodiments, the mechanical membrane in the etch release process controlmonitor may be electrically shorted to the ITO/chrome layers.

The extent of etching may be measured using the process control monitorsdescribed above by visually observing the process control monitors or byelectronically imaging the process control monitors, such as by using aCCD camera, and then computationally analyzing the image so that themeasuring is automated. In some embodiments, posts in the processcontrol monitor may be used as a vernier for determining the extant ofetching. For example, posts may be formed in the process control monitorhaving a known distance from each other. The number of posts along aline from the center of the hole may then be used to approximate thedistance. In some embodiments, a higher density of posts than formed inthe interferometric modulators may be used to provide a more precisemeasurement. Those of skill in the art will appreciate many other shapesand structures that may be used to measure the extent of etching.

Interferometric Modulator Process Control Monitors

In one embodiment, the interference properties (e.g., the spectrum ofreflected light) of interferometric modulators may be determined byusing a process control monitor that consists of an interferometricmodulator with enhanced stability. Such process control monitors may beconstructed so that the mechanical membrane is resistant to movement,and therefore fixed in position, forming a static etalon. In oneembodiment, such process control monitors may be formed by using asubstantially transparent dielectric layer (e.g., an oxide layer) inplace of the sacrificial layer. The reflective mechanical membrane willthus rest against the dielectric layer and be in a fixed position. Sucha process control monitor may advantageously be manufactured separatelyfrom a display interferometric modulator array so that a thicker oxidelayer can be deposited than is deposited during typical interferometricmodulator manufacture.

In another embodiment, a process control monitor is formed that may bemanufactured by the same material depositions as a displayinterferometric modulator array. For example, as depicted in FIG. 12, aprocess control monitor 204 may be constructed that comprises a higherdensity of posts 220 than is found in the interferometric modulatorarray 202. The higher density of posts 220 provide increased positionalstability to the mechanical membrane that they support. Accordingly,even under application of an electric potential (e.g., less than about10 volts, 15 volts, or 20 volts), the mechanical membrane in the processcontrol monitor 204 will resist moving toward the ITO layer and thereby,reflect the same spectrum of light. As used herein, by “posts” it ismeant any intermittent structure that may be used to support amechanical membrane. Accordingly, it is intended that “posts” include“point” structures consisting essentially of a vertical linearstructure. It is also intended that “posts” include structuresconsisting essentially of a strip of vertical material, also known asrails.

Process control monitors having stable mechanical membranes, such asdescribed above, may be used to optimize manufacturing to produceinterferometric modulators that will reflect a desired spectrum oflight. Furthermore, such process control monitors provide a quick checkof the success of a manufacturing process. In some embodiments, where amanufacturing process produces an array of interferometric modulatorsthat reflect different colors (e.g., for use in a polychromaticdisplay), multiple process control monitors as described above may beused, each reflecting the corresponding color. Alternatively, a singleprocess control monitor may be formed having different regions whereeach region has posts having a different height than other regions.Thus, each region will reflect a different color light.

Thickness Process Control Monitors

Still another type of process control monitor is used to measure thethickness of each layer deposited during processing. In one embodiment,thickness process control monitors are manufactured such that a singlestep is formed from the substrate to the top of the process controlmonitor. The step height of the single step will thus correspond to thecombined thickness of the all the layers of the process control monitorat the location of the step. Non-limiting examples of layers that may bedeposited include the ITO and chrome layers, the oxide layers, thesacrificial layer, planarization on the sacrificial layer, mechanicalmembrane layer on the oxide layers, and mechanical membrane layers onthe sacrifical layer on the oxide layers.

In another embodiment, multiple layer process control monitors areformed such that a stack with a stair-step pattern profile may beformed. The step heights will correspond to the thickness of one or moredeposited layers. For example, the resulting process control monitor mayhave a structure similar to that in FIG. 14. The process control monitorin FIG. 14 contains each of the layers deposited during manufacturing ofan interferometric modulator, such as the one depicted in FIG. 9. Theprocess control monitor provides steps corresponding to the thickness ofthe ITO layer 154, the chrome layer 150, the oxide layers 156/158, thesacrificial layer 160, the planar material 162, and the mechanicalmembrane, 164/166. The thickness of each step may be measured in asingle sweep of an appropriate thickness measuring technique rather thanhaving to measure each layer in a separate process control monitor. In anon-limiting example, a stylus-based surface profiler (e.g., aprofilometer), such as available from KLA-Tencor may be used to measurestep heights by a single sweep of the stylus and thus quickly determinethe thickness of each layer deposited in a particular interferometricmodulator manufacturing run. The stair step pattern reduces the naturalbounce encountered when using profilometers and thereby improvesaccuracy as compared to sweeping across each layer individually. Thoseof skill in the art will recognize that any combination of layers may beused in a multi-stair step pattern. Thus, not all layers depositedduring manufacture of interferometric modulators need to be included.

Another embodiment of a thickness process control monitor is depicted inFIG. 15. This process control monitor also has a stair-step profile;however, the stair-step pattern formed does not monotonically increasein height. One advantage of such a pattern is that the step heights maybe formed to more closely correspond to actual thicknesses present insome interferometric modulators. In addition to layers discussed above,the process control monitor of FIG. 15 also contains a dark mask layer275. The dark mask layer 275 may be used in interferometric modulatorsto inhibit reflection from some static structures such as posts andrails. In this embodiment, an additional oxide layer 277 may bedeposited above the dark mask layer 275.

Step 300 in FIG. 15 corresponds to the combined thicknesses of all ofthe oxide layers (277, 156, and 158) and the dark mask 275. This stepmay be compared with step 302 to determine the thickness of the darkmask 275. The absolute height of step 304 provides the combinedthickness of the oxide layers 277, 156, and 158 and the ITO 154 andchrome 150 layers. Comparison with step 302 provides the thickness ofthe combined ITO 154 and chrome 150 layers. Step 306 provides thethickness of the oxide layers 156/158 that are deposited on top of theITO 154 and chrome 150 layers. Step 308 corresponds to the thickness ofthe mechanical membrane 164/166. The absolute height of step 308 willalso approximate the combined thicknesses of material when aninterferometric modulator is in an actuated state with the mechanicalmembrane 164/166 collapsed on top of the oxide layer 158. Step 310corresponds to the combined thicknesses of the mechanical membrane164/166 and the planar material 162. Comparison with step 308 may beused to determine the thickness of the planar material 162. Step 312corresponds to the thickness of the sacrificial layer 160. Finally, step314 corresponds to the thickness of the planar material 312. Theabsolute height of step 314 also corresponds to the position of themechanical membrane 164/166 when an interferometric modulator is in anunactuated state.

In some embodiments, polychromatic interferometric modulator displaysare formed. One such polychromatic display uses interferometricmodulators having different gap depths to reflect different colors. Forexample, interferometric modulators having three different gap depthsadapted to reflect predominantly red, green, or blue colors may beemployed. One method of forming such a polychromatic display is todeposit and pattern three sacrificial layers prior to deposition of theplanar material and mechanical membrane layers. The patterning of thesacrificial layers may be such that a single layer remains for one setof interferometric modulators, two layers remain for another set ofinterferometric modulators, and three layers remain for a final set ofinterferometric modulators. After deposition of the mechanical membraneand release etching, a smaller gap depth will be formed where the singlesacrificial layer was formed, a medium gap depth will be formed wheretwo sacrificial layers were formed, and a larger gap depth will beformed where three sacrificial layers were formed. FIG. 16 depicts athickness process control monitor that may be used to measure layerthicknesses formed during use of such a three-sacrificial layer process.In addition to sacrificial layer 160, sacrificial layers 279 and 281 arealso formed. Those of skill in the art will appreciate that thesacrificial layers 160, 279, and 281 may be deposited sequentially orthey may be deposited in a different order if liftoff or etch backtechniques are utilized. Step 350 corresponds to the combinedthicknesses of all of the oxide layers (277, 156, and 158) and the darkmask 275. This step may be compared with step 352 to determine thethickness of the dark mask 275. The absolute height of step 354 providesthe combined thickness of the oxide layers 277, 156, and 158 and the ITO154 and chrome 150 layers. Comparison with step 352 provides thethickness of the combined ITO 154 and chrome 150 layers. The comparisonof step 356 with step 354 provides the thickness of the oxide layers156/158 that are deposited on top of the ITO 154 and chrome 150 layers.Step 358 corresponds to the thickness of the mechanical membrane164/166. The absolute height of step 358 also approximates the combinedthicknesses of material when an interferometric modulator is in anactuated state with the mechanical membrane 164/166 collapsed on top ofthe oxide layer 158. Step 360 corresponds to the combined thicknesses ofthe mechanical membrane 164/166 and the planar material 162. Comparisonwith step 358 may be used to determine the thickness of the planarmaterial 162.

Step 362 corresponds to the combined thickness of the mechanicalmembrane 164/166 and the single sacrificial layer 160. Comparison ofstep 362 with step 358 provides the thickness of the sacrificial layer160. The absolute height of step 362 also corresponds to the position ofthe mechanical membrane 164/166 when the interferometric modulatorhaving the smallest gap depth is in an unactuated state. The absoluteheight of step 364 corresponds to the combined height in aninterferometric modulator array over a post that is between twointerferometric modulators having the smallest gap depth. Comparison ofstep 364 with step 358 provides the height of the post. In a similarmanner, step 366 corresponds to the combined thickness of the mechanicalmembrane 164/166 and the first 160 and second 279 sacrificial layers.Comparison of step 366 with step 362 provides the thickness of thesecond sacrificial layer 279. The absolute height of step 366 alsocorresponds to the position of the mechanical membrane 164/166 when theinterferometric modulator having the medium gap depth is in anunactuated state. The absolute height of step 368 corresponds to thecombined height in an interferometric modulator array over a post thatis between two interferometric modulators having the medium gap depth.Comparison of step 368 with step 358 provides the height of the post.Step 370 corresponds to the combined thickness of the mechanicalmembrane 164/166 and first 160, second 279, and third 281 sacrificiallayers. Comparison of step 370 with step 366 provides the thickness ofthe third sacrificial layer 281. The absolute height of step 370 alsocorresponds to the position of the mechanical membrane 164/166 when theinterferometric modulator having the largest gap depth is in anunactuated state. The absolute height of step 372 corresponds to thecombined height in an interferometric modulator array over a post thatis between two interferometric modulators having the largest gap depth.Comparison of step 372 with step 358 provides the height of the post.

The process control monitor of FIG. 16 provides for the accuratemeasurement of the gap depths produced by a particular interferometricmodulator manufacturing process. Measuring the cumulative height of thesacrificial layers corresponding to the medium and large gap depthinterferometric modulators provides a more accurate indication of theresulting gap depth than would be obtained by measuring the individualthicknesses of the three sacrificial layers. If the three layers weremeasured separately, local variation in the thickness of each layerwould be compounded when the thicknesses are added together to obtaintotal gap depths. In contrast, the process control monitor of FIG. 16provides single measurements of the combined thicknesses the sacrificiallayers, reducing errors introduced by local variances in each separatesacrificial layer.

In the embodiments of FIGS. 15 and 16, the mechanical membrane 164/166may be used to protect the sacrificial layer 160 in the process controlmonitors during the release etch. Accordingly, in some embodiments, thethickness process control monitors may be evaluated after release etch.In some other embodiments, thickness process control monitors may beevaluated prior to the release etch. If the results indicate a problemwith one or more layer thicknesses, the wafer may be scrapped prior tothe release etch, thereby saving time and money.

Those of skill in the art will recognize that many other stair-steppatterned process control monitors may be produced. It will also beappreciated that thickness process control monitors that contain lessthan all of the layers in a MEMS device may be constructed.

Although the invention has been described with reference to embodimentsand examples, it should be understood that numerous and variousmodifications can be made without departing from the spirit of theinvention. Accordingly, the invention is limited only by the followingclaims.

1. A method of monitoring the extent of etching of a first materialpositioned between and adjacent to two layers of other material duringmanufacturing of a micro-electro-mechanical system (MEMS), comprising:manufacturing a process control monitor that comprises the two layers ofother material and the first material disposed between and adjacent tothe two layers, wherein one of the two layers comprises a hole; exposingthe hole to an etchant; and optically detecting a distance from thecenter of the hole to where the etchant has etched away the firstmaterial, whereby the distance is indicative of the extent of etching ofthe first material.
 2. The method of claim 1, wherein the opticaldetecting comprises detecting reflectance of light from a side of theprocess control monitor where the first material and the two layers ofother material are deposited during manufacturing of the process controlmonitor.
 3. The method of claim 1, wherein the optical detectingcomprises detecting reflectance of light from a side of the processcontrol monitor opposite the side where the first material and the twolayers of other material are deposited during manufacturing of theprocess control monitor.
 4. The method of claim 1, wherein the MEMSincludes interferometric modulators and the first material is asacrificial layer that determines positioning of a mechanical membranerelative to other layers in the interferometric modulators.
 5. Themethod of claim 4, wherein the first material is molybdenum.
 6. Themethod of claim 4, wherein the first material is silicon.
 7. The methodof claim 1, wherein the hole is photolithographically created.
 8. Themethod of claim 1, wherein substantially different reflectivity in thevisible spectrum is detected where the etchant has etched away the firstmaterial as compared to where the etchant has not etched away the firstmaterial.
 9. The method of claim 1, wherein the optical detectingincludes visually observing the process control monitor.
 10. The methodof claim 1, wherein the optical detecting includes detecting lightreflected from the process control monitor with a camera.
 11. The methodof claim 1, wherein the optical detecting comprises visually observingthe distance.
 12. The method of claim 1, wherein the optical detectingis performed while exposing the hole to the etchant.
 13. The method ofclaim 1, wherein the optical detecting is performed after exposing thehole to the etchant.
 14. The method of claim 13, wherein the opticaldetecting is performed after removing the process control monitor froman etchant chamber.
 15. The method of claim 1, wherein the manufacturingcomprises forming a plurality of posts in the process control monitorthat support the separation of the two layers of other material afterremoval of the first material.
 16. The method of claim 15, wherein theposts are used as a vernier to determine said distance.
 17. A wafer,comprising: a plurality of structures comprising a sacrificial layer andat least one layer above and adjacent to the sacrificial layer, whereinthe structures become interferometric modulators upon removal of thesacrificial layer, wherein the at least one layer above and adjacent tothe sacrificial layer comprises a plurality of holes through which anetchant can reach the sacrificial layer; and a process control monitoralso comprising the sacrificial layer and the at least one layer aboveand adjacent to the sacrificial layer, wherein the at least one layerabove and adjacent to the sacrificial layer in the process controlmonitor comprises multiple holes, wherein the distance between the holesin the process control monitor is greater than the distance between theplurality of holes in said plurality of structures.
 18. The wafer ofclaim 17, wherein the distance between the holes in the interferometricmodulators and the process control monitor are such that the etchant cansubstantially completely remove the sacrificial layer in theinterferometric modulators before it can substantially completely removethe sacrificial layer in the process control monitor.
 19. The wafer ofclaim 17, wherein the at least one layer above the sacrificial layercomprises a mechanical membrane that includes a mirror.
 20. The wafer ofclaim 17, wherein the plurality of structures and the process controlmonitor are disposed on a substantially transparent substrate.
 21. Awafer, comprising: a plurality of structures comprising a sacrificiallayer and at least one layer above and adjacent to the sacrificiallayer, wherein the structures become interferometric modulators uponremoval of the sacrificial layer, wherein the at least one layer aboveand adjacent to the sacrificial layer comprises a plurality of holesthrough which an etchant can reach the sacrificial layer; and a processcontrol monitor also comprising the sacrificial layer and the at leastone layer above and adjacent to the sacrificial layer, wherein the atleast one layer above and adjacent to the sacrificial layer in theprocess control monitor comprises a single hole.
 22. The wafer of claim21, wherein the single hole has a diameter substantially the same as theplurality of holes.
 23. The wafer of claim 21, wherein the processcontrol monitor is adapted so that the etchant can substantiallycompletely remove the sacrificial layer in the interferometricmodulators before it can substantially completely remove the sacrificiallayer in the process control monitor.
 24. The wafer of claim 21, whereinthe distance between the single hole and the edges of the processcontrol monitor is greater than the distance between the plurality ofholes in said plurality of structures.
 25. The wafer of claim 21,wherein the at least one layer above the sacrificial layer comprises amechanical membrane that includes a mirror.
 26. The wafer of claim 21,wherein the plurality of structures and the process control monitor aredisposed on a substantially transparent substrate.
 27. A method ofmanufacturing a wafer having a micro-electro-mechanical system (MEMS)and a process control monitor structure, the method comprising: forminga plurality of structures, wherein forming the plurality of structuresincludes one or more material deposition and removal steps, wherein thestructures comprise a sacrificial layer and at least one layer above andadjacent to the sacrificial layer, wherein the at least one layer aboveand adjacent to the sacrificial layer comprises a plurality of holesthrough which an etchant can reach the sacrificial layer; simultaneouslyforming a process control monitor, wherein forming the process controlmonitor includes said one or more material deposition and removal steps,wherein the process control monitor also comprises the sacrificial layerand the at least one layer above and adjacent to the sacrificial layer,wherein the at least one layer above and adjacent to the sacrificiallayer in the process control monitor comprises multiple holes, whereinthe distance between the holes in the process control monitor is greaterthan the distance between the plurality of holes in said plurality ofstructures; and exposing the plurality of structures and the processcontrol monitor to an etchant.
 28. The method of claim 27, wherein theprocess control monitor is formed with a structure different from theplurality of structures by applying at least one different patterningstep.
 29. The method of claim 28, wherein the different patterning stepcomprises defining a pattern for the multiple holes in the processcontrol monitor.
 30. A wafer, comprising: a micro-electro-mechanicalstructure (MEMS); and means for measuring extent of etching of amaterial removed during manufacturing of the MEMS.
 31. The wafer ofclaim 30, wherein the means comprises a process control monitor.
 32. Thewafer of claim 30, wherein the MEMS comprises an interferometricmodulator.
 33. The wafer of claim 32, wherein the material comprises asacrificial layer that determines the depth of a gap in theinterferometric modulator.
 34. A process control monitor produced byprocess comprising: depositing at least three layers of material on topof each other; and forming a hole in the top layer of material.